The present invention relates to a current flowing direction switching circuit for switching a direction in which a current flows through a load.
A current switching circuit for switching a current flowing through a load is disclosed in, for example, Japanese Patent Kokai Publication JP-A-6-90523, which proposes the configuration of a bidirectional current regulating circuit comprising a first and second switches connected to an output of the current regulating circuit, having respectively second and first terminals which are grounded, and respective common terminals which are connected to first and second output terminals, respectively, to which a load of an electronic control equipment is connected whereby the first and second switches are simultaneously switched by a drive circuit, so that the direction of a current appearing at the first and second output terminals is switched in response to a signal which is applied to a control input terminal which is connected to an input of the drive circuit. Since only unidirectional output is required as an output of the current regulating circuit, the necessity of two power sources including positive and negative power sources is eliminated.
However, the following problems have been encountered in the course of the investigations toward the present invention. Namely, in order to change over the direction of a current flowing through a drive circuit, this prior art bidirectional current regulating circuit requires two-contact switches. If there is a variation in the operation time between two switches, the output of the current regulating circuit may be short-circuited to the ground (GND).
A transistor bridge circuit and a control circuit for controlling the same is disclosed in, for example, Japanese Patent Kokai Publication JP-A-62-104492, which teaches a circuit comprising a positive phase amplifier, inverting amplifier and NPN type transistors in which a voltage on a terminal of a current detecting resistor which is connected to a transistor bridge circuit is supplied to a subtracting circuit, said voltage having a polarity which is preset depending upon the direction of a current flowing through a motor, so that an eddy current which is induced in the motor is prevented from being induced irrespective of the direction of a current flowing through the motor. In this control circuit, a circuit for preventing the eddy current from occurring is complicated and is not capable of completely preventing the eddy current from occurring due to the variation in the operation time of the transistors when the operation speed is high.
An example of the configuration of a prior art current direction switching circuit is illustrated in FIG. 4. Referring now to FIG. 4, a two-terminal load (Z) 3 is connected at its one end to one end of a resistor R1 and a collector of a phototransistor Q2-1 which constitutes an NPN type photocoupler 5-1. The resistor R1 is further connected at the other end to a positive power source. The phototransistor Q2-1 has an emitter which is connected to a negative power source through an emitter resistor RE1. The load 3 is connected at the other end to one end of a resistor R2 and the collector of a phototransistor Q2-2 which constitutes an NPN type photocoupler 5-2. The resistor R2 is connected at the other end to a positive power source. The phototransistor Q2-2 has an emitter which is connected to a negative power source via an emitter resistor RE2. Reference numerals 6 and 7 denote drive circuits (buffer circuits) for driving the photodiodes D1 and D2, respectively.
If an input and output signals A and B assume Low and High levels, respectively, the photocoupler 5-2 could be turned off, a current would flow through a photocoupler 5-1, a current would flow from the positive power source through the resistor R2 and from the other end to the one end of the load 3, and a current would flow from the collector of the phototransistor Q2-1 through the emitter resistor RE1 to the negative power source. At this time, a current flows from the positive power source, also through the resistor R1 and the photo-diode Q2-1 to the negative power source.
If the input and output signals B and A assume the Low and High levels, respectively, the photocoupler 5-1 turns off and the photocoupler 5-2 turns on, a current would flow through diode D2 of the photocoupler 5-2, the phototransistor Q2-2 is turned on, a current would flow from the positive power source through the resistor R1 and from one end to the other end of the load 3, and a current would flow from the collector of the phototransistor Q2-2 constituting the photocoupler 5-2 through the emitter resistor RE2. At this time, a current flows from the positive power source, also through the resistor R2 and the photo-diode Q2-2 to the negative power source.
In such a manner in the prior art circuit as shown in FIG. 4, a current flows through any one of the resistors R1 and R2 which are on the side where one of the photocouplers 5-1 and 5-2 is turned on and an unwanted current other than a necessary current flowing through the load 3 will flow, resulting in an increase in power consumption.
Therefore, the present invention has been achieved in view of the above-mentioned problem. It is an object of the present invention to provide a current switching circuit in which its power consumption is reduced by preventing an unwanted current other than necessary current for a load from flowing. It is another object to provide a current switching circuit having a simplified circuit configuration. Still further objects of the present invention will become apparent in the entire disclosure.
In order to accomplish the above-mentioned object, in a first aspect of the present invention, a load having one end which is connected to the ground and the other end which is connected to one end of each of the switches, the first switch having the other end which is connected to a positive current source and the second switch having the other end which is connected to a negative current source.
In a second aspect of the present invention, a load is connected at one end to the ground, the load is connected at other end to a connection node between collectors of phototransistors of first and second photocouplers, emitters of the phototransistors which make up the first and second photocouplers are connected to positive and negative power sources via a first and second resistors, respectively, first and second control signals are input to the first and second photocouplers, respectively, and a direction in which current flows through the load is changed over by selectively turning on any one of the first and second photocouplers.
In a third aspect of the present invention, there is provided a circuit for switching over a direction in which current flows. The circuit comprises: a load having one end which is connected to the ground; and a first transistor and a second transistor, each having a drain which is connected to the other end of the load; wherein sources of the first and second transistors are connected to a positive power source and a negative power source through a first resistor and a second resistor, respectively; and wherein a first control signal and a second control signal are input to gates of the first and second transistors, respectively, and a direction of a current flowing through the load is changed over by selectively turning on any one of the first and second transistors.
In a fourth aspect of the present invention, there is provided a circuit for switching over a direction in which current flows, wherein a load is connected at its one end to the ground; the load is connected at the other end to a connection node between collectors of phototransistors of first and second photocouplers; emitters of the phototransistors each making up the first and second photocoupler, respectively, are connected to a positive power source and a negative power source via a first resistor and a second resistor, respectively; a first control signal and a second control signal are input to the first photocoupler and the second photocoupler, respectively; and a direction in which the current flows through the load is changed over by selectively turning on any one of the first and second photocouplers. The first and second control signals are connected to cathode terminals of light emitting diodes each making up the first and second photocouplers, respectively.